1. Technical Field
The present invention relates to methods for measuring colour. More particularly the present invention involves measuring the colour of samples printed on substrates containing brighteners.
2. Background Art
Optical brighteners are often used in the production of papers. Optical brighteners can improve the degree of whiteness of the paper (generally substrate) and reduce manufacturing costs.
Optical brighteners absorb light in the ultraviolet (UV) wavelength range from 320 to 410 nm and re-emit fluorescent light in the visible blue spectral range between 420 and 550 nm. The maximum of the fluorescent spectrum is between 430 and 440 nm.
The effect of optical brighteners and the resultant colour of the paper are influenced to a large degree by the spectral distribution of the illuminating light, primarily due to the ratio of the light levels in the UV and in the blue spectral range. The colour reproduced by printed samples is additionally influenced by the absorption behaviour of the colour coating on the paper substrate.
The non-linear behaviour of optical brighteners places high demands on colour measuring technology. The objectives of colour measuring technology are to obtain measurement values which correlate well with a specific visual observation condition with a defined lighting spectrum on the one hand. In terms of the process control and exchange of measurement data, on the other hand, it is important that different measuring devices output measurement values that are as far as possible identical on the same samples.
The current situation in colour measuring technology is satisfactory for printed samples on substrates containing no optical brighteners. If, however, optical brighteners are used, higher unsatisfactory variances are observed in colour measurement values.
In order to correctly adapt measuring devices for substrates containing brighteners, it is important that the lighting spectra in the devices have identical relative distributions in the UV range and in the blue spectral range. This is because higher variances between measuring devices occur primarily in the UV range.
Furthermore, In order to obtain compatibility with visual observation, it is necessary that the lighting conditions in the device be identical to those for visual observation. In technical terms, this congruence is very difficult to achieve since external light conditions are variable.
Current hand-held colour measuring devices, such as the SpectroEye made by Gretag Macbeth AG, use a glow lamp as the light source. The SpectroEye device has a filter wheel in the measuring optics. The lighting spectrum and receiver characteristic can be modified using different measuring filters. To obtain good conformance of the device, it is recommended that brightened samples be measured using the in-built UV blocking filter. This filter eliminates the UV element of the illuminating light so that the optical brightener is not able to generate any fluorescence. This eliminates the requirements for exact control of the lighting spectrum. The problem with the UV block filter method, however, is the fact that the measurement values do not match the real observation conditions because typical light sources usually contain a UV element and thus excite the brighteners.
It is possible to obtain exact measurement results with what is known as the bi-spectral measuring method. A bi-spectral measuring device has a monochromator in the lighting optics and a spectral analyser in the receiver channel. The measurement takes place sequentially. A complete reflection spectrum is measured for every lighting wavelength and stored in matrix format. The resultant reflection spectrum for the sample is determined by multiplying the matrix by a vector which contains the spectral optical energy distribution of the required light type. There is no restriction on this measuring technique; however, the sequential measuring procedure is time-consuming. Furthermore, the measurement technique is expensive to set up and is therefore impractical for use on an industrial scale. Examples of bi-spectral measuring systems include the BFC-450 device made by the Labsphere company and the CM-3800 made by Minolta.
US patent specification No. 6844931 describes a colour measuring system with lighting using variable light-emitting diodes (LED) and a spectral analyser in the receiver. The LED light source comprises a plurality of differently coloured, white and UV LEDs. The individual LEDs can be individually activated so that the spectral light distribution can be electronically adapted to a desired spectrum. The spectral reflection factor of the sample is then determined with an individual measurement using the desired lighting spectrum.
US patent application No. 2007/0086009 A1 (EP 1775565 A1) describes a method of determining the reflection spectrum of the sample using a double measurement. In the first measurement, the reflection spectrum is determined using the known UV block filter technique. In the second measurement, only UV light is used for the lighting and the fluorescence spectrum is measured separately. By adding the correctly weighted spectra accordingly, it is then possible to output the reflection factor for any type of excitation light. One disadvantage of this method is the fact that every sample has to be measured twice.
Against the background of this prior art, an objective of the invention is to provide a method of measuring the colour of brightened samples which makes it possible to determine the total spectral reflection factor of the sample for desired target light types quickly and obviates the need for double measurements, which can be implemented easily and inexpensively using existing colour measuring devices at the same time. The objectives underlying the invention are achieved as described herein.